Gas flow characterization in additive manufacturing

ABSTRACT

A method of characterizing gas flow within a housing includes: positioning one or more gas flow sensors in the housing; introducing a gas flow into the housing; using the one or more gas flow sensors to generate two or more gas flow measurements at spaced-apart locations within the housing; and recording the two or more measurements to create a gas flow map.

BACKGROUND OF THE INVENTION

This invention relates generally to additive manufacturing, and moreparticularly to apparatus and methods for gas flow characterization inadditive manufacturing.

Additive manufacturing is a process in which material is built uplayer-by-layer to form a component. Additive manufacturing is limitedprimarily by the position resolution of the machine and not limited byrequirements for providing draft angles, avoiding overhangs, etc. whichare required by casting. Additive manufacturing is also referred to byterms such as “layered manufacturing,” “reverse machining,” “directmetal laser melting” (DMLM), and “3-D printing”. Such terms are treatedas synonyms for purposes of the present invention.

One type of additive manufacturing machine is referred to as a “powderbed” machine and includes a build chamber that encloses a mass of powderwhich is selectively fused by a laser to form a workpiece. The buildchamber is enclosed in a housing that typically includes provisions fora flow of shielding gas therein. The shielding gas is used to transferheat away from the surface of the power bed, and to prevent vaporizedpowder from condensing on the surface of the workpiece.

One problem with prior art additive manufacturing machines is that thegas flow velocity varies over the build surface and throughout the buildchamber. Specifically, the gas flow decelerates as it passes over thesurface, because of normal pressure and friction losses. The velocitymay also be inconsistent in a direction perpendicular to flow. Becauseof this variation, the gas flow rate may be acceptable in one locationbut too high or too low in another.

BRIEF DESCRIPTION OF THE INVENTION

At least one of these problems is addressed by a method ofcharacterizing gas flow in an additive manufacturing process.

According to one aspect of the technology described herein, a method ofcharacterizing gas flow within a housing includes: positioning one ormore gas flow sensors in the housing; introducing a gas flow into thehousing; using the one or more gas flow sensors to generate two or moregas flow measurements at spaced-apart locations within the housing; andrecording the two or more measurements to create a gas flow map.

According to another aspect of the technology described herein, anapparatus for characterizing the gas flow within a housing includes atleast one gas flow sensor configured to generate two or more gas flowmeasurements at spaced-apart locations within the housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the followingdescription taken in conjunction with the accompanying drawing figuresin which:

FIG. 1 is a schematic, partially-sectioned front elevation view of anexemplary additive manufacturing machine;

FIG. 2 is a schematic, partially-sectioned side elevation view of themachine of FIG. 1;

FIG. 3 is a schematic, perspective view of a build platform useable withthe machine of FIG. 1; and

FIG. 4 is a schematic, perspective view of a gantry useable with themachine of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1 illustratesschematically an additive manufacturing machine 10 suitable for carryingout an additive manufacturing method. Basic components of the machine 10include a table 12, a powder supply 14, a recoater 16, an overflowcontainer 18, a build platform 20 surrounded by a build chamber 22, adirected energy source 24, and a beam steering apparatus 26, allsurrounded by a housing 28. Each of these components will be describedin more detail below.

The table 12 is a rigid structure defining a planar worksurface 30. Theworksurface 30 is coplanar with and defines a virtual workplane. In theillustrated example it includes a build opening 32 communicating withthe build chamber 22 and exposing the build platform 20, a supplyopening 34 communicating with the powder supply 14, and an overflowopening 36 communicating with the overflow container 18.

The recoater 16 is a rigid, laterally-elongated structure that lies onthe worksurface 30. It is connected to an actuator 38 operable toselectively move the recoater 16 along the worksurface 30. The actuator38 is depicted schematically in FIG. 1, with the understanding devicessuch as pneumatic or hydraulic cylinders, ballscrew or linear electricactuators, and so forth, may be used for this purpose.

The powder supply 14 comprises a supply container 40 underlying andcommunicating with the supply opening 34 and an elevator 42. Theelevator 42 is a plate-like structure that is vertically slidable withinthe supply container 40. It is connected to an actuator 44 operable toselectively move the elevator 42 up or down. The actuator 44 is depictedschematically in FIG. 1, with the understanding that devices such aspneumatic or hydraulic cylinders, ballscrew or linear electricactuators, and so forth, may be used for this purpose. When the elevator42 is lowered, a supply of powder “P” of a desired composition (forexample, metallic, ceramic, and/or organic powder) may be loaded intothe supply container 40. When the elevator 42 is raised, it exposes thepowder P above the worksurface 30. Other types of powder supplies may beused; for example powder may be dropped into the build chamber 22 by anoverhead device (not shown).

The build platform 20 is a plate-like structure that is verticallyslidable below the build opening 32. It is connected to an actuator 46operable to selectively move the build platform 20 up or down. Theactuator 46 is depicted schematically in FIG. 1, with the understandingthat devices such as pneumatic or hydraulic cylinders, ballscrew orlinear electric actuators, and so forth, may be used for this purpose.When the build platform 20 is lowered into the build chamber 22 during abuild process, the build chamber 22 and the build platform 20collectively surround and support a mass of powder P along with anycomponents being built. This mass of powder is generally referred to asa “powder bed”, and this specific category of additive manufacturingprocess may be referred to as a “powder bed process”.

The overflow container 18 underlies and communicates with the overflowopening 36, and serves as a repository for excess powder P.

The directed energy source 24 may comprise any device operable togenerate a beam of suitable power and other operating characteristics tofuse or melt the powder P during the build process, described in moredetail below. For example, the directed energy source 24 may be a laser.

The beam steering apparatus 26 may include one or more mirrors, prisms,electromagnets, and/or lenses and may be provided with suitableactuators, and arranged so that a beam “B” from the directed energysource 24 can be focused to a desired spot size and steered to a desiredposition in plane coincident with the worksurface 30. For purposes ofconvenient description, this plane may be referred to as a X-Y plane,and a direction perpendicular to the X-Y plane is denoted as aZ-direction (X, Y, and Z being three mutually perpendicular directions).The beam B may be referred to herein as a “build beam”.

A basic build process for a workpiece W using the apparatus describedabove is as follows. The build platform 20 is moved to an initial highposition. The build platform 20 is lowered below the worksurface 30 by aselected layer increment. The layer increment affects the speed of theadditive manufacturing process and the resolution of the workpiece W. Asan example, the layer increment may be about 10 to 50 micrometers(0.0003 to 0.002 in.). Powder “P” is then deposited over the buildplatform 20 for example, the elevator 42 of the supply container 40 maybe raised to push powder through the supply opening 34, exposing itabove the worksurface 30. The recoater 16 is moved across theworksurface to spread the raised powder P horizontally over the buildplatform 20. Any excess powder P drops through the overflow opening 36into the overflow container 18 as the recoater 16 passes from left toright. Subsequently, the recoater 16 may be moved back to a startingposition. The leveled powder P may be referred to as a “build layer” andthe exposed upper surface thereof may be referred to as a “buildsurface”.

The directed energy source 24 is used to melt a two-dimensionalcross-section or layer of the workpiece W being built. The directedenergy source 24 emits a beam “B” and the beam steering apparatus 26 isused to steer a focal spot of the build beam B over the exposed powdersurface in an appropriate pattern. A small portion of exposed layer ofthe powder P surrounding the focal spot, referred to herein as a “weldpool” 52 (best seen in FIG. 2) is heated by the build beam B to atemperature allowing it to melt, flow, and consolidate. As an example,the weld pool 52 may be on the order of 100 micrometers (0.004 in.)wide. This step may be referred to as fusing the powder P.

The build platform 20 is moved vertically downward by the layerincrement, and another layer of powder P is applied in a similarthickness to the first layer. The directed energy source 24 again emitsa build beam B and the beam steering apparatus 26 is used to steer thefocal spot of the build beam B over the exposed powder surface in anappropriate pattern. The exposed layer of the powder P is heated by thebuild beam B to a temperature allowing it to melt, flow, and consolidateboth within the top layer and with the lower, previously-solidifiedlayer.

This cycle of moving the build platform 20, applying powder P, and thendirected energy melting the powder P is repeated until the entireworkpiece W is complete.

The machine 10 and its operation are as representative example of a“powder bed machine”. It will be understood that the principlesdescribed here are applicable to other configurations of powder bedmachines, as well as other machines utilizing a protective gasenvironment.

The housing 28 serves to isolate and protect the other components of themachine 10. The housing 28 is generically representative of anyenclosure, chamber, or similar structure which is effective to create aclosed or semi-closed environment. For example, a room of a buildingcould serve as a housing. During the build process described above, thehousing 28 is provided with a flow of an appropriate shielding gaswhich, among other functions, excludes oxygen from the buildenvironment. To provide this flow, the machine 10 may be coupled to agas flow apparatus 54, seen in FIG. 2. The exemplary gas flow apparatus54 includes, in serial fluid flow communication, a variable-speed fan56, a filter 58, upper and lower inlet ducts 60 and 62 respectively,communicating with the housing 28, and a return duct 64 communicatingwith the housing 28. All of the components of the gas flow apparatus 54are interconnected with suitable ducting and define a gas flow circuitin combination with the housing 28.

The composition of the gas used may similar to that used as shieldinggas for conventional welding operations. For example, gases such asnitrogen, argon, or mixtures thereof may be used. Any convenient sourceof gas may be used. For example, if the gas is nitrogen, a conventionalnitrogen generator 66 may be connected to the gas flow apparatus 54.Alternatively, the gas could be supplied using one or more pressurizedcylinders 68.

Once the gas flow apparatus 54 and machine 10 are initially purged withgas, the fan 56 is used to recirculate the gas through the gas flowcircuit in a substantially closed loop, so as to maintain the positivepressure described above, with additional added makeup gas added asneeded. Increasing the fan speed increases the velocity and flow rate ofgas in the gas flow circuit; conversely, decreasing the fan speeddecreases the velocity and flow rate of gas in the gas flow circuit. Asan alternative to recirculation, the gas flow apparatus 54 could operatein a total loss mode; for example instead of the gas flowing through thereturn duct 64 and back to the fan 56, it could simply be vented toatmosphere after passing over the build chamber 22. In the illustratedexample, the thermal mass of the gas provides a heat transfer function,however an optional heat exchanger (not shown) could be incorporatedinto the gas flow apparatus 54.

The upper inlet duct 60 is positioned near the top of the housing 28.During operation it provides a first stream or flow of gas (see arrow“G1”) to keep particulates away from the beam steering apparatus 26 andother optical components of the machine 10.

The lower inlet duct 62 is positioned near the bottom of the housing 28.During operation it provides a section stream or flow of gas (see arrow“G2”). As seen in FIG. 1, the lower inlet duct 62 has an elongated shape(for example rectangular) and discharges gas across the width of thebuild chamber 22. For reference purposes the width of the build chamber22 may be considered parallel to the “X” direction. As shown in FIG. 2,the edge of the build chamber 22 closest to the upper inlet duct 62 isreferred to as a “leading edge” 70, and the opposite parallel edge isreferred to as a “trailing edge” 72. For reference purposes the lengthof the build chamber (i.e. distance from leading edge 70 to trailingedge 72) may be considered parallel to the “Y” direction.

The second gas flow G2 has two functions. First, it is used to effectheat transfer and carry heat away from the surface of the uppermostbuilt layer within the build chamber 22. Second, during the buildprocess, some of the powder P is vaporized. This vapor can cool andcondense on the surface of the workpiece W, in turn causing anundesirable surface roughness or “recast” layer. Part of the second gasflow G2 is used to carry away the vapors and/or condensate.

It has been demonstrated that the gas flow velocity varies over thesurface of the build chamber 22. For example, the gas flow deceleratesas it passes over the surface parallel to the Y direction, because ofnormal pressure and friction losses. It may also be inconsistent in theX direction. Overall, the flow pattern may have complex characteristicsin the X, Y, and Z directions. The specific gas flow pattern will varyfrom machine-to-machine and can vary over time for one machine becauseof wear, filter plugging, or similar causes.

If the gas flow velocity over a particular location is too high, it candisturb the powder in the build chamber 22. If the gas flow velocity istoo low, it will provide insufficient heat transfer and vapor removal,resulting in measurably worse surface finish and mechanical properties.Because of the complex nature of the flow pattern described above, thegas flow can be acceptable in some parts of the build chamber 22 andunacceptable in others.

In such circumstances, simple single-point gas flow measurement may notbe sufficient to ensure adequate flow over the entire surface of thebuild chamber 22. Accordingly, means may be provided to characterize thegas flow within the housing 28, more specifically positional “mapping”of the gas flow, in two or three dimensions.

To enable gas flow characterization, the machine 10 may be provided withat least one gas flow sensor. Any type of sensor operable to generate asignal indicative of a gas flow measurement may be used. As used herein“gas flow measurement” refers to any measurement that quantifies gasflow. Examples of gas flow measurements include but are not limited tovelocity, dynamic pressure, volume flow rate, or mass flow rate.Nonlimiting examples of gas flow sensors include mechanical orsolid-state anemometers (for example a hot-wire anemometer, sonicanemometer, or laser Doppler anemometer), pitot tubes or otherdifferential pressure-based devices, or combinations of sensors operableto quantify flow (e.g. speed-density systems).

In the example shown in FIG. 3, a modified build platform 120, otherwisesimilar to build platform 20 described above, is provided with an arrayof spaced-apart gas flow sensors 74 in communication with its uppersurface 122. The location of the gas flow sensors 74 are thus fixed andknown. In operation, each gas flow sensor 74 generates an independentgas flow measurement. This permits positional mapping of the gas flowover the build platform 20, relative to the X and Y directions.Alternatively, the gas flow sensors 74 could be positioned at otherfixed locations within the housing 28.

In the example shown in FIG. 4, a gantry 76 is disposed above the buildplatform 20. The gantry 76 has a frame 78 with first, second and thirdactuators 80, 82, and 84, to drive a mount 86 in X, Y, and Z-directions,respectively. The mount 86 carries a gas flow sensor 88. The structureof the gantry 76 may be configured so as to minimize any gas flowdisturbances. For example, the frame 78 may be constructed from slenderrods, rails, or other similar elements. In operation, the gas flowsensor 88 is used to generate a gas flow measurement at multiplelocations. This permits positional mapping of the gas flow over thebuild platform 20, relative to the X, Y, and Z directions. The gantry 76may be considered as generally representative of a “sensor support”.

An example of a gas flow characterization process is as follows. Thehousing 28 is provided with appropriate gas flow sensors as describedabove. This would typically be done in an empty condition (i.e. nopowder or workpiece present). If integral sensors are used as shown inFIG. 3, the build plate 120 may be raised to an uppermost position. Forcost reasons, the build plate 120 may be installed for the gas flowcharacterization process, then removed and replaced with a standardbuild plate 20. This allows the build plate 122 be reused for multiplemeasurements. If the gantry 74 is used, it would be positioned over thebuild plate 20 and the housing 28 would be closed. The housing 28 wouldthen be prepared by using the gas flow apparatus 54 to purge air out ofthe machine 10 and allow the gas flow to reach steady state condition.Recording of gas flow measurements may then take place. If the gantry 74is used, the mount 86 would be moved to a plurality of differentpositions and a measurement taken in each position. The resulting datamay include a plurality of gas flow measurements, with correspondingpositional information. This information may be referred to collectivelyas a “gas flow map”. The gas flow map may be stored, for example, as amatrix, table, or electronic data file.

The gas flow map may be used for various purposes. For example, it maybe used for machine qualification. In this process, the gas flow sensorswould be used to characterize the gas flow and produce a gas flow mapbefore the machine 10 is used for the first time. This gives a baselinefor subsequent measurements, and also gives the user information aboutthe machine 10. For example, a specific machine 10 may be known to havea particular flow pattern which may require higher than average gas flowsettings to achieve acceptable flow patterns during the build process.The baseline gas flow map may be compared to a predetermined standardgas flow map, and the machine 10 adjusted such that the baseline gasflow map matches or more closely approximate the standard gas flow map.

As another example, the method may be used for machine calibration. Inthis process, the gas flow sensors described above would be used tocharacterize the gas flow and produce a gas flow map at regularintervals, for example every three to six months. The series of gas flowmaps could be compared to the baseline gas flow map and/or to eachother. This could help identify a change in the machine characteristics.For example if a gas duct becomes clogged, the gas flow map wouldchange. Corrective action could take the form of machine maintenance orrepairs. Alternatively, process parameters such as gas flow rate couldbe modified in subsequent builds (manually or automatically) tocompensate for machine degradation. For example, prior to a build, theflow could be mapped. If the map does not match the predeterminedstandard or baseline gas flow map, then adjustments could be made tomachine parameters.

The operation of the apparatus described above including the machine 10and gas flow apparatus 54 may be controlled, for example, by softwarerunning on one or more processors embodied in one or more devices suchas a programmable logic controller (“PLC”) or a microcomputer (notshown). Such processors may be coupled to the sensors and operatingcomponents, for example, through wired or wireless connections. The sameprocessor or processors may be used to retrieve and analyze sensor data,for statistical analysis, and for feedback control.

The method described herein has several advantages over the prior art.In particular, it provides consistent, adequate gas flow whileminimizing the flow of gas. This has the potential to reduce workpiecevariation and scrap rate, improve part quality, and monitor thecondition of the machine 10.

The foregoing has described an apparatus and method for gas flowcharacterization in an additive manufacturing process. All of thefeatures disclosed in this specification (including any accompanyingclaims, abstract and drawings), and/or all of the steps of any method orprocess so disclosed, may be combined in any combination, exceptcombinations where at least some of such features and/or steps aremutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings) may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of the foregoingembodiment(s). The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying potential points of novelty, abstract and drawings), orto any novel one, or any novel combination, of the steps of any methodor process so disclosed.

What is claimed is:
 1. A method of characterizing gas flow within ahousing, comprising: positioning one or more gas flow sensors in thehousing; introducing a gas flow into the housing; using the one or moregas flow sensors to generate two or more gas flow measurements atspaced-apart locations within the housing; and recording the two or moremeasurements to create a gas flow map.
 2. The method of claim 1 whereinthe housing encloses an additive manufacturing apparatus comprising:means for depositing a powdered material within the housing; and adirected energy source operable to selectively fuse powdered material inthe presence of the gas flow.
 3. The method of claim 1 wherein the gasflow is allowed to reach a steady state condition before generating thegas flow measurements.
 4. The method of claim 1 wherein the at least onegas flow measurement is gas flow rate or gas velocity.
 5. The method ofclaim 1 wherein a plurality of gas flow sensors are disposed in fixedlocations in the housing.
 6. The method of claim 1 wherein one or moresensors are mounted on a sensor support configured to move the one ormore sensors within the housing.
 7. The method of claim 6 furthercomprising: using the sensor support to position a selected one of theone or more gas flow sensors at a first position within the housing;generating a first gas flow measurement using the selected gas flowsensor; using the sensor support to move the selected gas flow sensor toa second position within the housing; and generating a second gas flowmeasurement using the selected gas flow sensor.
 8. The method of claim 1further comprising comparing the gas flow map to a predeterminedstandard and adjusting the gas flow so as to match the predeterminedstandard.
 9. The method of claim 2 further comprising, subsequent tocreating the gas flow map: performing within the housing an additivemanufacturing process by using the directed energy source to selectivelyfuse the powdered material to form a workpiece, in the presence of thegas flow, wherein the gas flow rate is controlled with reference to thegas flow map.
 10. The method of claim 9 further comprising repeating oneor more times the steps of creating a gas flow map and performing anadditive manufacturing process; and comparing a later one of the gasflow maps to an earlier one of the gas flow maps.
 11. The method ofclaim 10 further comprising, after comparison of the gas flow maps,adjusting the gas flow so as to match the earlier one of the gas flowmaps.
 12. An apparatus for characterizing the gas flow within a housing,comprising at least one gas flow sensor configured to generate two ormore gas flow measurements at spaced-apart locations within the housing.13. The apparatus of claim 12 wherein the at least one gas flow sensoris configured to measure gas flow rate or gas velocity.
 14. Theapparatus of claim 12 where a plurality of gas flow sensors are disposedin fixed locations in the housing.
 15. The apparatus of claim 12 wherethe a plurality of gas flow sensors are mounted on a plate-like buildplatform disposed within the housing.
 16. The apparatus of claim 12wherein one or more sensors are mounted on a sensor support such thatthe one or more sensors are moveable in at least two mutuallyperpendicular directions within the housing.
 17. The apparatus of claim12 wherein the housing encloses an additive manufacturing apparatuscomprising: means for depositing a powdered material within the housing;and a directed energy source operable to selectively fuse powderedmaterial in the presence of the gas flow.